Florida prosecutors face significant hurdles building a case against the man who killed Trayvon Martin, a legal expert has told the Guardian.

Jonathan Turley, a professor at George Washington University and a defence attorney, said the case had several "murky" elements which are likely to prove difficult for the prosecution, including within the 911 tape which appears to suggest Zimmerman was the one in pursuit of Martin.

Turley said the evidence was not nearly as conclusive of guilt as many people have assumed. "Some parts of the 911 tape would work to the disadvantage of Zimmerman, others to his advantage," he said. "It would be a very difficult case for the prosecution without more evidence."

He added: "We haven't seen any of the forensic evidence, which is the key."

It has him ignoring requests not to follow Martin but it includes a portion where he is saying that Martin is approaching him. We have heard reports that Zimmerman was bleeding as the result of a struggle. Zimmerman was a big individual and was armed. It's difficult to believe because of his size and the fact that he was armed that he felt in imminent threat of death. But lawyers could point to the tape showing that Zimmerman believed Martin might be armed. That Martin was moving towards him and checking him out. All of that is likely to be playing to a jury with the question of intend and whether he had a reasonable fear of serious injury or death.

At one point in the tape, Zimmerman refers to Martin's hand in his waistband.

Turley believes that the case is going to turn on classic criminal defence elements under common law, where an individual is allowed to use lethal force if they fear injury or death.

"You lose that defence if you are the aggressor or if you do not have a reasonable basis for fear or serious harm or death. Even if he is not the aggressor there will remain the question of escalation or confrontation."

The key difficulty, said Turley, is the "maddening gap" of evidence at the critical time of confrontation.

"At what point does a physical struggle justify the use of lethal force? If I was the defence counsel in the case, the first thing I would do is the trajectory of the bullet, how close it was to Martin, where it was fired."

Turley said the racial element is unclear. In the 911 tape, Zimmerman says "He looks black" and then says "he's a black male".

"The Zimmerman family have denied any racial element. On the tape he initially appears not to know the race, but then later does. All of these elements are murky."

Nevertheless, Turley believes there was enough evidence to arrest Zimmerman and to secure an indictment.

"Police would have been on solid grounds to arrest him. You have an unarmed teenager and someone who does not comply with a request that he not follow Martin.

"That doesn't mean there's enough evidence to secure a conviction."

Even under Florida's Stand Your Ground law, Turley said, Zimmerman, if indicted, would still have to demonstrate he had a reasonable fear of death or serious injury.